Since char is mainly composed of carbon and hydrogen, decreased amounts of carbon and hydrogen are released from char-forming materials to the gas phase as combustible gaseous products. Furthermore, since thermal conductivity of char is generally much lower than that of a polymer, a char layer acts as an excellent thermal insulation layer to protect virgin polymer underneath and as barrier to the passage of these gases. However, the detailed chemical reaction steps to form char as well as the chemical structure of char are not well understood. Studies are urgently needed to understand these reactions so as to better enhance the char formation rate and its amount.
As a thermal wave penetrates into the interior of a polymer, a highly complex generation and transport of degradation products occurs from the interior of the polymer outward through a strong viscosity gradient. The viscosity gradient has a significant influence on transport behavior. It appears that the transport process of the sub-surface degradation products supplies a majority of degradation products to the sample surface. For char-forming materials, sub-surface degradation products, which are dominant in this case, are transported through the many cracks that form in a hard and porous char. However, if an intumescent char is formed instead of hard, porous char, the transport of the sub-surface degradation products becomes more complex. The effectiveness of the thermal insulation of a char appears to depend on its physical structure. An intumescent, foamy char tends to have better insulation characteristics than a hard, brittle, dense char.
An important macroscopic transport process for thermo-plastic polymers is melting and dripping during burning. Since an aircraft interior consists essentially of a floor, two walls, and a ceiling, if thermoplastic materials on the walls and the ceiling are heated to well above their glass transition temperatures or melt temperatures (for crystalline polymers) by the thermal radiation coming through an opening such as a door, there could be a significant amount of melting and dripping of interior materials. Although such behavior might be helpful in certain fire scenarios, here it appears to increase the hazard; melting and dripping from wall and ceiling panels may interfere with the evacuation of passengers and crew, as well as enhancing fire growth.
Ignition occurs after sufficient amounts of combustible degradation products reach the gas phase. Generally, non-charring polymeric materials (e.g., polyolefins) degrade below 400°C which is too low to produce nonpiloted ignition (Kashiwagi, 1981). However, char-forming materials (e.g., phenolics) could reach a surface temperature high enough (due to low thermal conductivity of the char) to allow nonpiloted ignition. However, the limiting requirement to cause piloted ignition is a sufficient supply of gaseous, combustible degradation products. Therefore, piloted ignition tends to occur much earlier than nonpiloted ignition (Kashiwagi, 1981).
After ignition, the growth of fire is determined by the flame-spread characteristics of materials. The total heat release rate of a fire is determined by the integral of the burning surface area times the local burning rate per unit surface area. Local heat release rates are the result of a complex coupling between condensed and gas-phase phenomena. Flame spread depends on continued generation and transport of degradation products to the flame. The generation rate of combustible degradation products is determined by the heat-and mass-transport processes and also by the chemical degradation reactions.
The relationship between materials chemical structure, composition, and fire performance is presently understood on a general, empirical basis (Weil, 1995; Wilkie, 1995). Previous sections of this report described the burning process as it applies to aircraft interiors and identified important properties and analysis methods to understand combustion and to evaluate materials performance. Based on this understanding of the critical issues and requirements and the structure performance relationships, approaches for significantly improving fire resistance by either affecting the pyrolysis rate or degradation product composition, or by changing the gas-phase reaction rates can be identified. This section outlines potential advances in fire-safe materials, including improvements in organic materials, organic/inorganic materials, inorganic preceramic polymers, innovative material systems, and advanced additive approaches. In addition to the advances described in this report, future prospects for improved fire-resistant materials are described in the proceedings of the conference that the committee hosted (NRC, 1995). Reviews are included that describe research and trends in fire resistant polymers (Wilkie, 1995), additive concepts (Weil, 1995), and inorganic and organometallic polymers (Zeldin, 1995).
Research to develop a fundamental understanding of chemical structure is most likely to contribute to the development of more-fire-resistant products and reduced flammability